1. Field of the Invention
The present invention relates to an ink jet recording apparatus and an ink jet recording method for recording by ejecting ink through a nozzle whenever a driving signal is applied.
2. Related Background Art
The ink jet recording apparatus has advantages that it is comparatively easy to reduce the size of a recording head and it is possible to record a high resolution image at high speed at less running cost.
Particularly, since a heater element, for the recording head according to a bubble-jet method in which ink is ejected by using thermal energy, which gives heat to ink, can be formed on a substrate by deposition through a semiconductor manufacturing process, the recording head can be manufactured in a very small size.
In a recording apparatus according to the bubble-jet method (thermal ink jet method) in which ink is ejected by using such thermal energy, as the total number of recorded sheets increases, the number of times of starting the recording apparatus increases, and the number of times of ink ejecting exceeds a predetermined threshold value, disconnection breakdown has frequently occurred in a heater (heater element) of a recording element, preventing ink from being ejected thereby.
In the bubble-jet method, ink is ejected with repeating processing in which a bubble is generated, grown and shrunk, based on heating by a heater element. One of the causes for the above disconnection breakdown is the breakdown of the heater element (which may include a protective film), which breakdown is caused by centering of an impact force on a fixed location of the heater, which force is caused when a physical impact (hereinafter called “cavitation”) is applied to the heater element upon defoaming of the bubble. The physics of cavitation will be explained, referring to drawings.
FIGS. 15A, 15B, 15C and 15D are explanatory views of the physics of cavitation. In FIG. 15A, 150 shows an exemplary view of an ink flow channel and 8C is an ejecting heater (heater element). When energy is applied on the ejecting heater 8C, the temperature of ink near the surface of the ejecting heater is raised to cause a change in state from liquid to gas through phase transition and a bubble 152 is generated. The pressure level of foaming gas at a start point of foaming is raised to a level approximately exceeding 10 atmospheres and, thereafter, the pressure in the gas is reduced to 1/100 atmospheres or less when the bubble reaches the maximum foaming point only by inertia force (FIG. 15A). Then, shrinking force is generated by the lower pressure in the gas and defoaming is started (FIG. 15B).
Refilling of ink is started along with shrinkage of the gas and inertia force is generated in ink once ink is started to move. In the middle of defoaming, the pressure in the bubble is in a state of negative pressure relative to the atmospheric pressure and the shrinking force is applied on the bubble in the shrinking direction by which the bubble itself is shrunk. From a certain point in time, the shrinkage advances while the bubble is pushed and crushed by the inertia force of the ink and the pressure in the gas becomes extremely high. When the gas is compressed to the limit (FIG. 15C), the gas cannot exist in a vapor phase and defoaming processing is completed (FIG. 15D) after phase transition to a liquid phase.
The process advances with extremely high speed. When the above-described recording head was driven under the above-described conditions, the time required from the point when the bubble reached the maximum foaming point to the point at completion of defoaming was approximately 5 μs. Here, the pressure level is instantaneously changed from an extremely high state to the normal pressure (ink pressure open to the atmosphere) at phase transition of the final step in the above process. The impact force caused by the pressure change on the surface of the ejecting heater is cavitation. In order to prevent reduction in the lifetime with regard to disconnection in the heater caused by the cavitation, a protective film for anti-cavitation has been required to be provided on the heater.
However, the protective film for anti-cavitation causes reduction in the transmission efficiency of the thermal energy from the heater to ink and the efficiency of the energy used for ejecting is decreased. More particularly, when the film thickness is increased to improve the strength, the energy efficiency is further remarkably reduced. Thereby, there has been a problem to be solved, the problem being that the temperature of the recording head itself is easily raised to an extremely high temperature.
The present invention has been made, considering the above-described problems, and an object of the invention is to provide an ink jet recording apparatus and an ink jet recording method, by which stable image quality can be obtained together with an effectively extended lifetime with regard to the disconnection and without decreasing the efficiency of energy used for ejecting, because the deterioration of recorded images caused by disconnection in heaters is controlled without acceleration of deterioration of the heater element and without adverse effects owing to use environment, the deteriorated state of the heater element, scattering in recording heads at manufacturing, and the like.